To Accurately Calculate Earth’s Circumference

How was a man living over 2,000 years ago able to accurately calculate Earth’s circumference? How could he even know that Earth is spherical? How can some people today believe that our planet is flat when everyone has seen pictures of Earth taken by satellites in space?

Those may be too many questions at once. Let’s try looking at them one at a time. The first man to accurately calculate Earth’s circumference in about 240 B.C. was a Greek polymath named Eratosthenes. He was a brilliant mathematician, geographer, astronomer, music theorist, and poet, and his calculations were amazingly accurate.

A myth originated from a fiction work by nineteenth-century American author Washington Irving and other authors. Irving wrote a less-than-accurate “biography” of Christopher Columbus. In it, the Spanish authorities questioned Columbus’ plan to sail west to Asia by going east because they thought the ships would drop off the edge of a flat Earth. The truth is that European scholars at that time knew that our planet is a sphere. In fact, Columbus did too, but he believed that it was 25% smaller than it actually is. He should have paid more attention to Eratosthenes.

Ancient Greek scholars, as early as the fifth century B.C., recognized the spherical nature of the planet based on observations. (We have talked about that before.) So Eratosthenes set out to accurately calculate Earth’s circumference. Let’s look at a simplified description of how he did it.

Eratosthenes was the librarian of the famous library of Alexandria in Egypt. Syrene was a city 5,000 stadia south in what is now Aswan, Egypt. (5000 stadia was approximately 800 km or 497 miles.) At noon on the summer solstice, Eratosthenes placed a rod vertically into the ground in Alexandria. At the same time, his assistant had placed a rod of the same length at Syrene. Since Syrene is very close to the Tropic of Cancer, where the Sun is directly overhead at noon on the solstice, the rod did not leave a shadow. In Alexandria, the rod produced a shadow of 7 degrees, which is 1/50 of the circumference of a circle. That means Earth’s circumference would be 50 times the distance from Alexandria to Syrene. Multiplying 50 times 5,000 stadia results in 250,000 stadia for Earth’s circumference. Depending on exactly how long a stadion was, that measurement is accurate to within -2.4% to +0.8%.

That answers the first two questions, but what about people today who believe the Earth is flat when there is plenty of evidence otherwise? That is an example of people believing what they want to believe and refusing to accept the evidence. There is a connection here to the so-called war between science and faith, and it relates to what we read in Romans 1:20. More on that tomorrow.

— Roland Earnst © 2023